It happens for the button "New Task" in task view and for "New Event" in the today pane on Windows with Aero theme.
Steps to reproduce:
set all the calendars in read only mode
the mentioned buttons get disabled and the labels text get the grayed out look, but the icons don't change.

Created attachment 667335[details][diff][review]
patch
The patch fixes.
I've also tried to assign the class "toolbarbutton1 calbar-toolbarbutton-1" (the same of the buttons in the toolbar) instead of assign directly the opacity. It works but the text in New task button gets a wrong position.